1. Field of the Invention
The present invention relates to a shock absorbing structure. More particularly, it relates to a shock absorbing structure suitable for shoes to be used in daily life, sports shoes for jogging, tennis, and golf shoes and the like, shoes for medical usage, mattresses for beds, cushioning sheets for chairs, floor coverings, packing or wrapping materials for protecting products and other shock absorbing materials.
2. Description of the Related Art
Heretofore, there have been used, as shock absorbing materials for shoes, foamed sheets from an ethylene-vinyl acetate copolymer (hereinafter referred to as "EVA"), natural rubber, synthetic rubber, polyurethane and the like, resin structures constructed like a branched tree as disclosed in Japanese Kokohu Publication (examined) 43224/1972, polyurethane solid elastomers having a high shock-damping capacity, bags filled with viscous materials or gas, and the like. These materials, however, have defects in absorbing shock which occurs when the foot contacts the ground, supporting the body or receiving impact force at the time of striking the ground with the foot without restricting motion of the body in walking and exercising.
It is ideally required that the shock absorbing material absorb the impact force which occurs in walking or exercising. It is also required against back and forth motion or right and left motion which occur in exercising, that the material firmly fix the foot on the ground and prevent the right and left motion as far as possible, and that the material simultaneously produce a reacting force for the next motion. It is further required that the shock absorbing material protect the ankle. These requirements are contrary to each other. For example, if a soft material is employed as the shock absorbing material, it often twists the ankle and may often injure it. Accordingly, conventional shock absorbing materials are poor in durability, cushioning characteristics, producibility of reaction force, protection of an ankle and a combination thereof.
For example, in view of durability, the EVA foam and the tree-like resin structure are too high in change rate in comparison with the other materials, such as, synthetic rubber, natural rubber, polyurethane, synthetic rubber foams and the polyurethane rubber foams (See Table 3). They are also poor in appearance after a compression test. These results indicate that the EVA foam and the tree-like resin structure reach a permanent compression set of an early stage and are poor in durability. Thus, they are not suitable in long time usage. The polyurethane foam and the synthetic rubber foam has a small change rate in comparison with the EVA foam and has good durability. They, however, are not good in change rate in comparison with non-foamed materials and are poor in appearance. They, therefore, are not adequate in view of durability.
In view of shock absorbing properties, the EVA foam, as shown in FIG. 10, has a high stiffness at an initial (light) weight load, but has a smaller stiffness at a heavy weight load. Thus, the foam exhibits good shock absorbing properties in heavy load conditions, e.g. in running, but poor in light load conditions, e.g. in walking. The polyurethane and synthetic rubber foams show shock absorbing properties, as shown in FIG. 11. If the foams have a high stiffness, the degree of deformation at an initial (light) weight load is very small to result in low shock absorbing properties when walking. In order to enhance the shock absorbing properties, if the foams having a low stiffness are employed, then their deformation degrees are inherently too large to protect the ankle in running. The tree-like resin structure, as shown in FIG. 12, is low in initial stiffness and high in stiffness at a heavy weight load due to its structure. Thus, this structure is preferred, because the shock absorbing properties are good in walking and the deformation degree is small in a heavy weight load condition, such as in running. This structure, however, has defects in that the foot is not fixed and moves in the shoe in which the foot is put, in such sports as need an accelerated speed, i.e. track events and tennis. It is also necessary that the structure be made from a hard resin, in order to fully enjoy its technical effects. Shoes thus made, however, have a bad feeling when the foot is put in them. When the tree-like structure is incorporated in the shoes, flashes which occur from cut portions have to be removed and the resulting structure also has to be covered with cloth. Accordingly, this structure has some difficulties in producibility.
The polyurethane solid elastomers having a high shock-damping capacity and the bags filled with viscous fluid have excellent shock absorbing properties, but are short of reaction forces necessary for the next motion and insufficient to protect the ankles. The bags filled with gas have a defect that the gas gradually leaks out after a long time.
In order to resolve the above mentioned defects, it is also proposed to combine two or more materials mentioned above. The combination makes a toe and a heel of a shoe thick, which makes it heavy. The process for preparing the shoes is also complicated.
As discussed above, the conventional shock absorbing materials have the following defects: (1) durability is poor, and (2) Each one of the materials does not have fully satisfactory shock absorbing properties. For example, the material has proper shock absorbing ability even in a light weight load in walking but, if in a heavy weight load in running, it has an increased stiffness and a little deformation. Further, reaction forces necessary for the next motions are produced and twist of the ankle does not occur. (3) In the case where two or more of the materials are combined to obtain desirable properties, weight and thickness increases and its production process becomes complicated. It often has a defect in durability.